Check at the minicube F3 guide I wrote as it covers how to bind a spectrum reciever. Essentially within betaflight you need to use the CLI to activate bind mode and then once bound you need to disable it and it will save the bind setting on your satellite​ reciever.

The Minicube F3 flight stack is built to be the best stack for your micro brushless FPV quadcopter. It includes everything you need in an FPV flight controller, plus more. However, there is generally little information around explaining everything...

My only other suggestion would be to double check you are using the correct bind mode, (DSMX, DSM2 etc..). But to be honest I am not a expert here as have not used spektrum gear for a good few years now. I use Frsky gear myself.